Washing hands in hot water offers no more hygienic benefits than using room temperature water—and significantly adds to greenhouse gas emissions worldwide.

If Americans en masse used tepid water instead of warm or hot to wash their hands, it could prevent annual greenhouse gas emissions totaling the equivalent of the United States’ lead industry or the entire output of a nation the size of Barbados, researchers say.

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“Although the perception that hot water is more hygienic is based in some factual evidence . . . there are few, if any, hygienic benefits of using warm or hot water to wash one’s hands,” writes Amanda Carrico, research assistant professor at the Institute for Energy and Environment at Vanderbilt University.

“It is true that heat kills bacteria; however, the level of heat required to neutralize pathogens is beyond what is considered safe for prolonged human contact.”

In addition to global warming emissions, the use of elevated temperatures can cause skin irritation, which can lead to more bacterial colonization, not less.

Some soap may emulsify better with warmer water, but for most routine hand washes any such gains are irrelevant if hand washers follow a proper regimen of scrubbing, rinsing, and drying.

Other kinds of heated water use have been targeted by global warming researchers as potential ways to reduce greenhouse gas emissions, including laundry and dish washing.

No health benefits

The study on hand washing, published in the International Journal of Consumer Studies,shows that 69 percent of 510 people surveyed believed that hand washing with warm or hot water is more effective than room-temperature water.

“The average respondent reported using warm or hot water 64 percent of the time,” Carrico writes. “Although the choice of water temperature during a single hand wash may appear trivial, when multiplied by the nearly 8 billion hand washes performed by Americans each year, this practice results in over 6 MMt (million metric tons) of CO2 eq (equivalent emissions) annually.”

The temperature used during hand washing would be a good candidate for a public information campaign, the study says.

“Multiple federal and state organizations recommend using elevated water temperatures, in some cases citing its superior cleaning ability as the reason.

“There is no doubt that the intention of these organizations is to promote a behavior that is vital to protecting public health; however, the preponderance of evidence . . . suggests this recommendation provides no health benefit.”

The public should be encouraged to use a “comfortable” water temperature when washing their hands. In colder climates warm water may be most comfortable.

“Although the degree of temperature was not measured in the current study, these data suggest that those who hold accurate beliefs do choose against selecting warm or hot water at the faucet on many occasions.”